1. Field Of the Invention
This invention pertains to the field of hydraulic controls for automatic transmissions. More particularly the invention pertains to a control system for a torque converter lockup clutch.
2. Description of the Prior Art
U.S. Pat. No. 4,876,923 describes a hydraulic control circuit in which torque converter lockup clutch is pressurized from a source of regulated line pressure through a lockup clutch control valve, which directs pressure to either of two passages to apply and release the lockup clutch. The control valve is supplied with control pressure from a solenoid-operated valve that produces high and low fluid pressure states corresponding to commands to engage and release the lockup clutch. A second control pressure, opposing the effect of the solenoid-operated control pressure, is applied to the control valve when the vehicle operator manually selects certain gear ratios, principally the lowest forward and reverse gear ratios. A line pressure regulator valve produces line pressure, whose magnitude is proportional to the magnitude of the control pressure representing a commanded engine torque output. The pressure produced when the vehicle operator manually selects low gear ratios is applied also to the main regulator valve, which produces a line pressure that varies linearly with engine torque control pressure and increases, in comparison to line pressure and other gear ratios, when low gear ratios are selected. A converter regulator valve limits the magnitude of line pressure directed by the main regulator valve to the converter clutch control valve.
If the solenoid control pressure, which represents a command for lockup clutch engagement or disengagement, is continually present, perhaps the result of an electrical failure or control system malfunction, the presence of the control pressure produced when the vehicle operator selects manually a low gear ratio will overcome the effect of the malfunction and release the torque converter lockup clutch. This action opens the torque converter and makes available to the transmission the torque multiplication capacity of the torque converter. When the transmission operates in its lowest gear ratios, the highest torque capacity of the transmission is assumed, therefore, in this mode of operation and with the torque converter controlled by the system of this invention, the torque multiplication capacity of the torque converter is assured regardless of the pressure state produced by the solenoid-operated control valve.
However, a lockup clutch can engage unintentionally due to several adverse operating conditions. A technique is required to prevent its inadvertent engagement despite the presence of these conditions.